In a vehicle whereupon an engine (internal combustion engine) is mounted, an automatic transmission automatically and optimally setting a gear ratio between the engine and drive wheels is known as a transmission for suitably transmitting torque and revolution speed generated by the engine to the drive wheels in accordance with a driving condition of the vehicle.
For example, a planetary gear type transmission setting a gear ratio (gear position) using a clutch, a brake, and a planetary gear apparatus, and a belt-type continuously variable transmission (CVT) performing stepless adjustment of a gear ratio exist as an automatic transmission mounted in a vehicle.
A belt-type continuously variable transmission has a belt wound around a primary pulley (input-side pulley) and a secondary pulley (output-side pulley) having a pulley groove (V-groove) and is configured such that a gear ratio is set in a stepless fashion by simultaneously expanding a groove width of the pulley groove of one of the pulleys and contracting a groove width of the pulley grove of the other pulley so as to continuously vary a winding radius (effective diameter) of the belt with respect to each of the pulleys. A torque transmitted in this belt-type continuously variable transmission is a torque corresponding to a load acting in a direction in which the belt and the pulleys are made to contact mutually, and therefore, in order to apply tension to the belt, the belt is clamped by the pulleys.
Furthermore, as explained above, a speed change is carried out in a belt-type continuously variable transmission by expanding and contracting the groove widths of the pulley grooves. Specifically, each of the primary pulley and the secondary pulley comprises a fixed sheave and a moveable sheave, and a speed change is carried out by moving the moveable sheave forwards and backwards in an axial direction using a hydraulic actuator provided at a rear face side thereof.
In this way, in a belt-type continuously variable transmission, the belt is clamped by the pulleys in order to apply tension to the belt, and in addition, the condition of clamping of the belt by the pulleys is changed in order to carry out a speed change. Accordingly, the groove width of the primary pulley and the groove width of the secondary pulley are simultaneously changed by delivering hydraulic pressure corresponding to a required torque as typified by engine load, etc. to the hydraulic actuator at the secondary pulley side so as to secure a necessary transmission torque capacity and by delivering hydraulic pressure for carrying out a speed change to the hydraulic actuator at the primary pulley side.
Furthermore, in a vehicle whereupon an automatic transmission is mounted, a fluid-type transmission apparatus such as a fluid coupling or torque converter, etc. is disposed between the engine and the automatic transmission. A fluid-type transmission apparatus provided with a lock-up clutch directly connecting an input side and an output side of the fluid-type transmission apparatus through frictional engagement using a hydraulic pressure of an operating oil exists as a fluid-type transmission apparatus.
Furthermore, in a vehicle whereupon this type of fluid-type transmission apparatus featuring a lock-up clutch is mounted, engagement and disengagement of the lock-up clutch is performed by controlling a hydraulic pressure made to act on the lock-up clutch with a hydraulic pressure (line pressure) of a hydraulic control system including hydraulic control of an automatic transmission used as a source pressure (for example, see patent documents 1 and 2). Specifically, in a case of a torque converter featuring a lock-up clutch, control of engagement and disengagement of the lock-up clutch is performed by controlling a differential pressure (lock-up differential pressure) between an engagement-side pressure chamber and a disengagement-side pressure chamber of the torque converter with a lock-up control valve using a line pressure as a source pressure.
Furthermore, in certain cases in the control of a lock-up clutch, deceleration lock-up control to control engagement of the lock-up clutch is performed upon deceleration with the accelerator off. With deceleration lock-up control of this type, in order to prevent stalling of the engine in a case of, for example, sudden braking of a vehicle, etc., it is possible to rapidly disengage the lock-up clutch by maintaining engagement of the lock-up clutch with the lowest possible hydraulic pressure (low-pressure engagement pressure within a range where slipping does not occur).    Patent Citation 1: JP H10-068462A    Patent Citation 2: JP H10-026220A    Patent Citation 3: JP H07-217713A